Four things to get right first
Map art uses the map palette, not the block colour
A map draws each block with a fixed colour from a small palette, and the shade depends on the height of the block next to it. Picking blocks by how they look in hand gives the wrong picture.
A circle in blocks is not a circle in maths
Rounding to the nearest block is what makes a shape look right or look lumpy. Use a generator and copy the row widths.
Count the materials before you start
A large build eats far more than you expect. Work the totals out first so you make one trip, not six.
Schematic formats are not interchangeable
.schem, .schematic, .litematic and structure blocks all store the same idea differently. Convert rather than hope.
Tools that do the work
Reference, straight from the game files
Read out of the game with Mojang’s own data generator, so it matches the version exactly.